Heterotrimeric G protein-mediated signal transduction is a tightly regulated event. All known G protein-coupled receptor (GPCR) mediated signaling pathways rely on multiple regulatory mechanisms in order to prevent inappropriate induction of the signal and to facilitate recovery during chronic stimulation (Gilman (1987) Ann. Rev. Biochem. 56:615–649, reviewed in Simon et al. (1991) Science 252:802–808; Conklin and Bourne (1993) Cell 73:631–641; Neer (1995) Cell 80:249–257; Rens-Domiano and Hamm (1995) FASEB J. 9:1059–1066). These regulatory mechanisms function at every level of the signaling cascade. Regulation of GPCR activation is believed to involve phosphorylation of the receptor C-terminus and subsequent receptor internalization (Palczewski and Benkovic (1991) Trends Biol. Sci. 16:387–391; Goodman et al. (1996) Nature 383:447–450; Chen and Konopka (1996) Mol. Cell. Biol. 16:247–257) though this does not appear to be a universal mechanism (Daunt et al. (1997) Mol. Pharm. 51:711–720). Known mechanisms of regulation of signal transduction at the level of the heterotrimeric G protein include receptor-mediated facilitation of GTP/GDP exchange on Gα (reviewed in Simon et al. (1991) Science 252:802–808; Conklin and Bourne (1993) Cell 73:631–641; Neer (1995) Cell 80:249–257; Rens-Domiano and Hamm (1995) FASEB J. 9:1059–1066) and enhancement of the intrinsic GTPase activity of Gα proteins by RGS-like proteins (reviewed in Berman and Gilman (1998) J. Biol. Chem. 273:1269–1272). Activation of PAKs, serine/threonine kinases that transduce signals from heterotrimeric G proteins to the MAP kinase cascade, has been shown to occur through interaction with either the small G proteins Cdc42 and Rac, or through interaction with heterotrimeric G proteins (reviewed in Sells and Chernoff (1997) Trends Cell Biol. 7:162–167). GPCR-coupled MAP kinase cascades and their downstream transcription factors, in turn, are regulated through phosphorylation/dephosphorylation cycles that may or may not require small G proteins (reviewed in Cobb and Goldsmith (1995) J. Biol. Chem. 270:14843–14846). Non-receptor activators of G-proteins have also been identified. These include both protein activators (Strittrnatter et al. (1993) Proc. Nat'l Acad. Sci., USA 90:5327–5331; Okamoto et al. (1995) J. Biol. Chem. 270:4205–4208; Sato et al. (1996) J. Biol. Chem. 271:30052–30060) and non-protein activators (summarized in Odagaki et al. (1998) Life Sciences 62:1537–1541.
Even with the identification of these diverse regulatory systems, an in-depth understanding of the temporal and spatial regulation of GPCR mediated signaling remains elusive. In fact, cellular variations in the efficiency and/or specificity of coupling observed for many specific receptor-heterotrimeric G protein complexes suggest the presence of additional unidentified, cell-specific regulators of the signaling process.